There is a broad range of physical and power radio frequency (RF) devices, such as gyrotrons, neutral beam generators and radars, that require high voltage, high-power variable voltage sources with high-frequency response and pulse-switching capabilities. High-power switching and voltage modulation for these applications has typically been accomplished by linear modulators based on high power vacuum electron tubes:
There are many disadvantages to these high-power linear modulators. In the active component of a linear modulator there is high thermal dissipation which dramatically increases when vacuum tubes are employed. The kilovolts of potential drop in the vacuum tubes can produce tens to hundreds of kilowatts of heat and require ancillary support systems the size of a small room.
High power vacuum electron tubes are expensive, fragile and are limited to about 150 A at 150 kV. These tubes require expensive support systems for control, cooling and crowbar protection. The tubes are bulky, sensitive to mechanical shock and magnetic fields, and have a finite life. Also, these devices cannot handle higher voltages envisioned for future gyroton or neutral beam systems due to the intrinsic parasitic oscillation and X-ray problems which presently limits the development of higher power systems.
Solid state devices that are commonly used, such as Insulated Gate Bipolar Transistors (IGBTs) and power Darlingtons, have limited dissipation capability and cannot be employed in high power, high voltage linear modulators.